linux-kernel - Re: [PATCH v1] dynamic_debug: add support for logs destination

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Message-ID: <ZSfyNgaHGGPiKqjN@phenom.ffwll.local>
Date:   Thu, 12 Oct 2023 15:18:46 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, jim.cromie@...il.com,
        Łukasz Bartosik <lb@...ihalf.com>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        "wayland-devel@...ts.freedesktop.org" 
        <wayland-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v1] dynamic_debug: add support for logs destination

On Thu, Oct 12, 2023 at 01:39:44PM +0300, Pekka Paalanen wrote:
> On Thu, 12 Oct 2023 11:53:52 +0200
> Daniel Vetter <daniel@...ll.ch> wrote:
> 
> > On Thu, Oct 12, 2023 at 11:55:48AM +0300, Pekka Paalanen wrote:
> > > On Wed, 11 Oct 2023 11:42:24 +0200
> > > Daniel Vetter <daniel@...ll.ch> wrote:
> > >   
> > > > On Wed, Oct 11, 2023 at 11:48:16AM +0300, Pekka Paalanen wrote:  
> 
> ...
> 
> > > > > - all selections tailored separately for each userspace subscriber
> > > > > (- per open device file description selection of messages)    
> > > > 
> > > > Again this feels like a userspace problem. Sessions could register what
> > > > kind of info they need for their session, and something like journald can
> > > > figure out how to record it all.  
> > > 
> > > Only if the kernel actually attaches all the required information to
> > > the debug messages *in machine readable form* so that userspace
> > > actually can do the filtering. And that makes *that* information UABI.
> > > Maybe that's fine? I wouldn't know.  
> > 
> > Well if you configure the filters to go into separate ringbuffers for each
> > session (or whatever you want to split) it also becomes uapi.
> 
> It's a different UAPI: filter configuration vs. message structure. I
> don't mind which it is, I just suspect one is easier to maintain and
> extend than the other.
> 
> > Also I'd say that for the first cut just getting the logs out on demand
> > should be good enough, multi-gpu (or multi-compositor) systems are a step
> > further. We can figure those out when we get there.
> 
> This reminds me of what you recently said in IRC about a very different
> topic:
> 
> 	<sima> swick[m], tell this past me roughly 10 years ago, would
> 	have been easy to add into the design back when there was no
> 	driver code yet 
> 
> I just want to mention today everything I can see as useful. It's up to
> the people doing the actual work to decide what they include and how.

I actually pondered this a bit more today, and I think even with hindsight
the atomic design we ended up with was probably rather close to optimal.

Sure there's a bunch of things that would have been nice to include, but
another very hard requirement of atomic was that it's feasible to convert
current drivers over to it. And I think going full free-standing state
structures with unlimited (at least at the design level) queue depth would
have been a bridge too far.

The hacks and conversion helpers are all gone by now, but "you can just
peek at the object struct to get your state" was a huge help in reducing
the conversion churn.

But it definitely resulted in a big price we're still paying.

tldr I don't think getting somewhere useful, even if somewhat deficient,
is bad.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch